Cranfield Institute of Technology - PhD, EngD, MSc, MSc by research theses, (CIT)

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Open Access
Acoustic emission pulse analysis: an integrated system for transducer calibration and signal processing
(1982-10) Oliveras, Juan M.; Bannister, R. H.
The present thesis covers the pioneering effort of establishing an infrastructure for conducting research into the analysis of Acoustic Emission signatures, with emphasis being placed upon the recovery of the true surface waveform for a given transient input. The topics covered in this work are: The p·rovi s ion of the necessary computer software for the handling and processing of the data. Here, a method is included for the correction of phase errors introduced by the sampling/multiplexing of ·the signals, which provides the basis for the implementation of data convolution.techniques in the frequency domain. J°he development of an experimental rig and source of excitation, later applied to the design of transducers. The development of a new transducer configuration, capable of producing a relatively flat frequency response (±7dB) over the frequency range 300 KHz - 2 MHz. The development of a procedure for the calibration of transducers, based upon a modified reciprocity method. The technique produces the calibration of transducers in relative terms, requiring a comple- . . mentary method to provide the reference level for the calibration. As a means of assessing the proposed techniques, the transducer calibration was applied to the de-convolution of pulses generated with a step forcing function, and the results compared with an equivalent theoretical model previously published.
Open Access
Aerodynamic surface pressure measurement in atmosphere and wind tunnel on a vertical axis wind turbine blade using pressure transducers
(1989-09) Oram, C. E.; Garside, A. J.; Hales, R. L.; Moss, J. B.
Blade aerodynamic loads during operation of a wind turbine are poorly understood. Better measurements of these forces are needed for the design of more cost effective turbines. Existing estimates have been inferred from strain gauges in the blade structure, but such measurements are confused by the complexity of the blade's structural response. This present work has pressure tapped the blade surface and recorded at high speed, giving a highly detailed picture of dynamic events and blade loads. A modular blade was constructed for the 6m diameter Rutherford Appleton Vertical Axis Wind Turbine and instrumented with 32 semiconductor pressure transducers. A flow probe using a further three transducers projected ahead of the blade during some of the tests. The instrumented module was also operated in the oscillating mounting of the University of Glasgow's 1.6m by 2.4m wind tunnel. 1760 revolutions of data were collected in atmosphere with blade speed ratios down to about 2.0 and with a variety of blade leanout angles. Some 118 runs of a variety of waveform types and Reynolds numbers from 0.3 to 1.1 million, were collected from the blade in tunnel. Analysis of transducer calibrations concluded that the novel temperature correction technique used allowed pressure readings to 40Pa or better with 95% confidence. Confidence in measured pressures has allowed use of leading edge stagnation point pressure and position as a measure of wind in the rotor flowfield and as a surrogate measure of blade angle of attack. Blade surface pressures and integrated forces show the progression of dynamic stall with increase in wind speed, the dominance of the upstream blade pass in usefulness at low and moderate windspeeds and the rough equality at high windspeeds. Comparisons of wind tunnel and on-rotor data show much lower force and moment coefficients in atmosphere. Rotor 'goodness1 ratios of average blade tangential force to maximum blade and rotor force coefficients peak at a blade speed ratio of about 2.8. Goodness ratios show a mixed pattern with blade leanout. Analysis of pressure coefficient traces through stall indicate a mixed stall type starting at mid chord, moving to leading edge and thence to trailing edge.
Open Access
Airblast atomization : the effect of linear scale on the mean drop size
(1978-12) El-Shanawany, M. S. M. R.; Lefebvre, A. H.
Stationary prefilming-cup airblast atomizers, in which the fuel is first spread into a thin cylindrical sheet and then exposed on both sides to high velocity air, have several important advantages over other common methods of fuel injection in their application to continuous combustion systems. This thesis reports the results of a detailed programme of research on airblast atomization directed primarily to the investigation of the effect of atomizer linear scale, or size, on the mean drop diameter of sprays and, to the provision of a comprehensive picture of the performance of thin-sheet airblast atomizers over a wide range of working conditions. Three geometrically similar atomizers giving cross-sectional areas in the ratio of 1 : 4 : 16 were used; covering the range of prefilming cup diameter, D, from 19.05mm. to 76.20 millimeters, and were capable of handling various liquids at flow rates from 0.003kg/s up to 0.225 kg/s, at fuel pressures below 33 x 10^ N/m^ (about 50 p.s.i.). . It was found that atomizer scale has an appreciable direct effect on atomization quality. The Sauter mean diameter of low liquid viscosity sprays increases with the 0.44 power of atomizer linear dimension (D), while for liquids of high viscosity this effect is slightly higher.
Open Access
Airline deregulation: an analysis under different regulatory and operating environments
(1989-07) Etemadi-Nejad Alamdari, F.; Cordey-Hayes, M.
Like other transportation modes the airline industry has a long history of government regulation. In recent years however the theoretical and empirical bases for air service regulatory schemes have been questioned by opponents of such policies. In Western Europe there have been pressures for relaxation of regulation of the air transport industry with respect to air fares, service frequency and market access. Because changes in such public policies are likely to have a major impact, there is a considerable interest in their possible implications. It is the objective of this study to explore the interactions between fare and service frequency in different competitive environments, and to examine the rivalrous behaviour of airlines and their possible equilibrium position under deregulation. The study also considers the effect of a competing mode (rail) on air carriers performance under various competitive conditions. The individual carrier's performance is quantified in terms of fare, service frequency, resulting generalised costs, market share and financial results. A competition model is developed by which the objectives set above are achieved. The study is divided into three parts. The first part provides a framework within which the competition model can be built. This part also generates the policy questions which must be addressed, should relaxation of regulation take place in Europe. The second part explains the structure of the competition model and its characteristics. The assumptions upon which the model is built and its limitations are also discussed. The model consists of three sub-models: the market share model, the costing model and the reaction model. In the third part, with the aid of the model the policy questions generated in the first part are addressed and the general implications for carriers operating under different competitive conditions are discussed. Finally the technical and the policy conclusions are discussed. It is generally concluded that: competition results in lower fares and a more efficient airline industry; the impacts of competition are not evenly distributed amongst markets, routes and carriers; and the competition on equal terms amongst carriers plays an important role in maintaining the effectiveness of deregulation.
Open Access
The application of electrolytic photoetching and photopolishing to AISI 304 stainless steel and the electrolytic photoetching of amorphous cobalt alloy
(1986-01) Talib, Talib Naji; Allen, David
The results presented cover broad aspects of a quantitative investigation into the elecrolytic etching and polishing of metals and alloys through photographically produced dielectric stencils (Photoresists). A study of the potential field generated between a cathode and relatively smaller anode sites as those defined by a dielectric stencil was carried out. Numerical, analytical and graphical methods yielded answers to the factors determining lateral dissolution (undercut) at the anode/stencil interface. A quasi steady state numerical model simulating the transient behavior of the partially masked electrodes undergoing dissolution was obtained. AISI 304 stainless steel was electrolytically photoetched in 10% w/w HC1 electrolyte. The optimised process parameters were utilised for quantifying the effects of galvanostatic etching of the anode as that defined by a relatively narrow adherent resist stencil. Stainless steel was also utilised in investigating electrolytic photopolishing. A polishing electrolyte (orthophosphoric acid —glycerol) was modified by the addition of a surfactant which yielded surface texture values of 70nm (Ra) and high levels of specular reflectance. These results were used in the production of features upon the metal surface through photographically produced precision stencils. The process was applied to the production of edge filters requiring high quality surface textures in precision recesses. Some of the new amorphous material exhibited high resistance to dissolution in commercially used spray etching formulations. One of these materials is a cobalt based alloy produced by chill block spinning. This material was also investigated and electro etched in 10% w/w HC1 solution. Although passivity was not overcome, by selecting suitable operating parameters the successful electro photoetching of precision magnetic recording head laminations was achieved. Similarly, a polycrystalline nickel based alloy also exhibiting passivity in commercially used etchants was successfully etched in the above electrolyte.
Open Access
The behaviour of polyurethane foam beams reinforced with thin walled steel sections for use in bus structures
(1980-08) Gaafar, Labib A.; Tidbury, G. H.
The thesis is directed to the theoretical determination of the behaviour of rigid skin polyurethane foam beams reinforced with thin walled steel sections. The aim is to enable the designer to predict the behaviour of bus body structure during accident situations in order to build safe buses. A literature search has revealed that very little work has been done in this particular field of research. Thus a preliminary test programme was made to investigate the beam behaviour in bending particularly at high deflection. The test results shown in Chapter 1 gave promising results for continuing the research. Chapter 2 is directed to elastic analysis of composite beams. One of the advanced methods to predict the elastic stiffness of sandwich beams is the interfacial shear method. This assumes an interfacial shear between the core and the faces of sandwich beams. So, by considering a similar assumption, i.e. an interfacial shear between the matrix and the reinforcement, an analysis was made to predict the stiffness of composite beams. Although this method highly overestimated the stiffness, it represents a good basis for the inelastic bending analysis which is more important for the present study. Chapter • 3 shows the inelastic bending analysis of composite beams. This analysis is based on TIMOSHENKO and JAMES curvature area method. The experimental verification of the theory is shown in Chapter 4. The theory shows good agreement with the experimental results. The shape of the reinforcement was optimised for uniaxial bending. This optimum composite section showed a weight saving when it was compared with equivalent energy absorbing rectangular steel tube. Impact tests were made using a pendulum designed specially to test cantilever beams at different speed and impact energies. The test results are presented in Chapter 5. These results showed that the composite beams behaviour is similar for both static and dynamic loading. To compare the composite beam with the constituent beams, the large deflection behaviour of thin walled channel section beams made of thin sheet steel has been investigated. The experiments consisted of cantilever bending tests with the beam loaded through the shear centre and through the centroid. When loaded through the shear centre the beam buckling took place in the compression flange at the root of the cantilever. When loaded through the centroid however, it was noted that the compression flange buckled at a fixed distance from the fixed end. The general theory of thin walled beams developed by Vlasov was applied to the problem and indicated that the maximum compression stress at the free edge of the flange would be a maximum at some distance from the fixed end. The value of the maximum compression stress obtained by the general linear theory was small and its position did not coincide with the experimental position. The Vlasov analysis has been modified to include the increase in the twisting moment due to the lateral deformation- of the beam along its length. Good agreement between the modified theory and experiment both for the position of the maximum compressive stress and for the twist of the cantilever at three points along its length. Because of the very low torsional stiffness of thin walled channel sections, the small deflection theory is only applicable for small bending loads applied through the centroid and the modified theory should be used for practical loading cases.
Open Access
Biosensor and bioelectrocatalysis studies of enzymes immobilized on graphite electrode materials
(1987-05) Schneider, B. H.; Higgins, I. J.; Turner, Anthony P. F.
The immobilization of glucose oxidase and lipoamide dehydrogenase on graphite electrodes was studied for their application in mediated bioelectrochemical systems. These are techniques where the electrons involved in an enzymatic redox reaction are either supplied by, or given to an electrode from the enzyme. In the biosensor mode, where a current is measured due to product oxidation, the immobilized enzyme may be used to monitor its substrate, for example a biosensor for glucose based on the enzyme glucose oxidase. In the bioelectrosynthesis mode, when a cathodic potential is applied to the electrode, electrons may be transferred to the enzyme via a mediator thereby driving a thermodynamically difficult reaction, such as the reduction of the coenzyme NAD+ . A glucose sensor based on glucose oxidase immobilized onto graphite foil electrodes which had been doped with the mediator dimethylferrocene was found to produce both a linear and rapid response to changes in glucose concentration in solution. The kinetics of this particular system were studied in depth in order to explain its behaviour. Results showed that the porous nature of the electrode material resulted in immobilization of the enzyme throughout a number of layers• Consequently, diffusion of glucose into the electrode became the rate limiting process at low glucose concentrations, which resulted in linear calibration curves. This was characterized by a substrate modulus, which was found to be sufficiently large to predict diffusional limitations. From this the effective rate of glucose diffusion within the electrode was estimated, and found to be up to 5 times smaller than that in free solution. The rapid response of the sensor, on the other hand, was due to the absence of any mass transfer limitations external to the electrode surface. When lipoamide dehydrogenase was employed in the same system it was found that complications arose due to coupling of NADH oxidation with the adsorbed mediator. By employing a less anodic potential of +150 mV vs. Ag/AgCl at NADH concentrations below 1 mM the detection of enzymatic oxidation of the coenzyme was achieved. Enzymatic reduction of NAD+ was achieved using lipoamide dehydrogenase immobilized onto graphite felt electrodes. This reaction was mediated by methyl viologen, with the enzyme-modified electrode used to reduce the mediator at a potential of -0.7 V ( vs. SCE ). In this fashion the reduced form of methyl viologen was recycled, thereby driving the reduction of NAD . By coupling this system to lactate dehydrogenase it was possible to effect the reduction of pyruvate.
Open Access
Characterisation of mixture preparation in lean premixed prevaporised combustor modules by planar laser fluorescence imaging and computational fluid dynamics modelling
(2001-04) Davoudi, Henrik; Greenhalgh, Douglas A.
In order to minimise the environmental impact of combustion emissions, radically new combustor concepts are under development. Lean premixed prevaporised (LPP) combustion has the greatest potential to achieve low emissions, particularly with respect to oxides of nitrogen. The role of fuel preparation is crucial in the success of the LPP combustor concept. Planar laser-induced fluorescence (PLIF) has been used to determine fuel placement and concentration, while computational fluid dynamics (CFD) has provided insight into the internal aerodynamics of two premixers. In order to determine the local equivalence ratios in a realistic device a calibration method, for the PLIF signal, has been derived. The vapour phase fluorescence of naphthalene, in the presence of oxygen, was calibrated against known conditions of temperatures and air-fuel ratios. This calibration was undertaken for an atmospheric and isothermal (non-combusting) flow. With regard to the small engine LPP module, PLIF measurements of the vapour fuel provided images of quantitative local equivalence ratios, at the exit of the premixer. In addition, based on the results of the liquid-phase measurements the evaporated fuel fraction was estimated. The computed droplet trajectories, within the premixing duct, highlighted the importance of the initial fuel dispersion from the injector. Two approaches were identified for improving the vaporisation and increasing the homogeneity of the air-fuel mixture. The large engine LPP module was investigated for several geometries and operating conditions. The evolution of the fuel dispersion, for those geometries, was characterised by PLIF images of the vapour fuel. In addition, the flame position inside the optical premixing duct was captured by video imaging for each geometry. 2D and 3D CFD simulations were performed to provide an insight into the aerodynamics of the premixer. These results were the basis of a schematic fuel jet breakup study, for two different injector configurations. Finally, the operational characteristics of this novel LPP module, featuring a conical centre-body, are discussed.
Open Access
Condition monitoring of slow speed rotating machinery using stress waves
(1998-06) Mba, David Uzo; Bannister, R. H.
Condition monitoring through the use of vibration analysis is an established and effective technique for detecting loss of mechanical integrity of a wide range and classification of rotating machinery. Equipment rotating at low rotational speeds present an increased difficulty to the maintenance engineer, since conventional vibration measuring equipment is not capable of measuring the fundamental frequency of operation, also, component distress at low operational speeds does not necessarily show an obvious change in vibration signature. This thesis presents a study of high frequency stress wave analysis as a means of detecting early stages of loss of mechanical integrity in slow speed machinery, the first of its kind. Investigations were centred on the Rotating Biological Contactor (RBC) which is used for sewage treatment in small communities and rotates between 0 .6 to 1 revolutions per minute. The mechanism of stress wave generation was the relative movements between mating components that were experiencing loss of mechanical integrity, for instance, the loss of tightening torque between clamped components. Development of the stress wave monitoring technique was accomplished by firstly, simulating a variety of mechanical faults on the test-rig, and secondly, by numerous ‘on-site’ tests on operational RBCs and their bearings. In application of this technique to operational RBCs and bearings, as well as cataloguing evidence of mechanical deficiencies associated with RBCs operated within the Severn Trent Water region, the author inspected 261 RBCs and covered over 35,000 miles, contributing to the uniqueness of this research programme. This research has provided Severn Trent Water limited with a PC-based monitoring system comprising an AE transducer, pre/post amplifiers, ADC board and software on which all diagnosis was undertaken. Furthermore, during the process of investigating reasons for the mechanical failures of the RBC, technical information was uncovered thereby assisting Severn Trent Water in perusing a more robust design of RBC. Moreover this work has assisted Severn Trent Water to maintain that envious reputation that over a number of years not one of the RBC sewage treatment works has failed its consent level as set by the Environmental Agency.
Open Access
The design and analysis of pipe diffusers for centrifugal compressors
(1997-12) Bennett, Ian; Elder, R. L.
Diffusers are necessary to convert the high velocity head of centrifugal compressor discharge flow into useful static pressure. One diffuser type that has shown a great deal of promise, particularly at high pressure ratios, are pipe diffusers. These have been successfully incorporated into centrifugal compressor stages in North America since the early 1970’s. With their commercial sensitivity, however, little design or detailed flow information has been published, and most of this literature is old, dating back twenty years to the design’s conception. This thesis is a compilation of pipe diffuser information, formed from the limited literature works and an intensive experimental parametric study. Two themes run through this thesis and are brought about by different reader needs. The first need is for design information. This must show the strengths and weakness of pipe diffusers and be of sufficient accuracy to potentially produce designs at the first attempt. The second need is to understand the flows within the diffuser itself, in order that through this knowledge further improvements can be made. Both of these requirements are addressed in this thesis. The studies are centred on, what was originally, a Rolls-Royce research centrifugal compressor impeller of 6:1 pressure ratio operating at 35,000 rpm with a tip speed of 533ms'1 consuming up to a nominal 1MW of power. Significant modifications to the original compressor assembly were made to allow modular changes of diffuser and permit detailed measurement access. Initially, a vaneless diffuser investigation was carried out with the primary aim of determining the flow entering the diffusers. This was followed by a diffuser throat sizing exercise as it is well recognised that the diffuser throat has a governing control over the complete stage performance. An exercise in changing the number of diffuser pipes was then undertaken, followed by a hybrid diffuser investigation incorporating an oval, rather than circular, cross-section. In all of these cases, detailed flow measurements were carried out using both conventional pneumatic instrumentation and a purposely developed unsteady pressure measurement data acquisition system. This system enabled detailed traverses of the diffuser throats to be undertaken. These unique measurements shed new light on the flow ‘seen’ by the pipe diffuser channel. Computational investigations using computational fluid dynamics (CFD) codes are carried out to compliment the experimental investigations.
Open Access
Design of an interactive finite element computer package for the analysis of the ride of a generalised off-road vehicle
(1987-02) Kamar, Elsayed Abdrabboh; Cookson, R. A.; El-Zafrany, A.
A versatile computer package, to evaluate ground vehicle ride comfort has been designed. Wheeled, tracked, and semitracked vehicles have been modelled. The emphasis has been placed upon the tracked vehicle suspension systems with the neatly ”standard" torsion bar arrangement operating with either telescopic or rotary-vane dampers. Substantial investigation of the four-bar linkage, associated with the rotary-vane damper, has been carried out and is presented here.The package includes linear ,nonlinear ,[2-V) ,and I3-Vj vehicle models. A new technique employing the finite element method (FEM) fat suspension system modelling has been developed. Several methods of numerical integration are available in the package .A new evident scheme based on a hypetosculatory Hermitian weighted residual of the second order for the solution of the system of differential equations which simulate the vehicle has been derived. Whenever necessary the error developed during the integration process was limited. The graphic display facility of digital computers has been used extensively in the present code. As an example, the damped eigen-value problem has been solved graphically. The graphical solution is simple, direct, and tellable compared to the analytical solution. The terrain configuration might be of either harmonic or atbitrary shape function. Two tolerence critetia have been employed fat tide comfort evaluation ,namely, the absorbed power critetion and the international standard organization ISO 2631 critetion.The two critetia are fully integrated into the package and operate automatically. All individual subroutines have been tested successfully for the purpose intended. Complete sectors have been validated by running various case studies. The results obtained from the package agree reasonably well with the analytical or experimental results. The package allows updating in response to new vehicle and vehicle-terrain technology and can be run on both time-skating or batch processing computer machines. The main package structute is independent of computer type.
Open Access
Development of a pollution prevention methodology for the metalcasting industry
(2001-01) Naylor, Tara J.; O'Callghan, P.
The metalcasting industry consists of small businesses that cast a wide variety of metals and use many different casting processes. The metalcasting industry is a large recycler of metal, however, metalcasting is energy intensive and produces significant air, land and water emissions. The industry is vulnerable to the cost of conforming to increasingly stringent environmental regulations. Improving environmental performance while keeping regulatory costs to a minimum is one of the key challenges of the industry. An adaptable methodology is required that is usable by small manufacturing units in order to comply with environmental regulation in a cost-effective manner. The processes used in the metalcasting industry were investigated. The available energy, waste, materials consumption data and the industry environmental effects were reviewed and evaluated. The methodologies used to reduce pollution to a minimum in other industries were evaluated for: applicability to environmental issues, suitability to both high production and jobbing shops, applicability to different casting methods, ease of use by foundry and environmental personnel, data availability. The methods assessed included: preliminary assessment methods, process flowsheets, mass and energy balances, manufacturing process understanding, process integration, pinch methods, exergy analyses, life-cycle assessment and total cost assessment. The developed methodology is presented in the form of a flowchart that combines an ordered sequence, with appropriate feedback loops, of the selected methods. The developed methodology was successfully tested at a working non-ferrous metalcasting facility. Fluoride chemicals, added to sand moulds and cores to prevent casting defects, were creating large quantities of hazardous solid waste and air emissions from the sand system. The application of the methodology resulted in the identification of a modified process that added the fluoride chemical in a surface coating only, thus reducing emissions by 99%. This methodology can be used to reduce waste, material and resource use in the metalcasting industry.
Open Access
The distribution of solids in a mechanically stirred liquid
(2000-09) Taylor, David J.; Brown, D. E.
The literature available on the effect of vessel geometry, particle properties and liquid properties on the distribution of solids in a mechanically stirred vessel has been examined. This has shown that there are wide discrepancies between different authors on each of these areas. Solids concentrations have been measured using an electrical conductivity probe at various positions in two stirred vessels 0.61m and 1.83m in diameter. Different impeller designs were examined using A 310s and Pitched Blade Turbines to distribute the solids in the vessel. Pitched Blade Turbines with different diameter ratios of D=T/3 and D=T/2 were used. The sand particles used-had diameters from 150 pm - 1050 pm. It has been shown that constant power per unit volume is a reliable design rule for: 1. The effect o f vessel scale 2. The effect o f impeller diameter 3. The effect o f impeller design on the distribution of solids in a mechanically agitated stirred standard vessel. It has also been shown that the solids concentration data presented here can be correlated on the basis of N mod = N (C )da{yPo~lD~5 J and the vessel operating conditions can be characterised by a design methodology based on its use. Two methods of modelling the data are presented which describe the system in terms of: 1. An array of oscillating spheres and 2. The 1-d sedimentation dispersion model. These methods were found to model the data with different degrees of success and were generally disappointing.
Open Access
Experimental and theoretical studies into the fatigue crack propagation behaviour of the 7075 aluminium-zinc alloy under constant and variable amplitude loading
(1991-03) Kyriacou, S. A; El-Zafrany, A.
This thesis describes both theoretical and experimental work performed in order to study the fatigue- crack propagation behaviour of the 7075 T7351 Aluminium - Zinc alloy, under constant and variable amplitude loading. For the theoretical aspects of the work. Static, Elastic and Elastic-Plastic Finite Element methods were utilised for the evaluation of residual stresses in the vicinity of the crack tip caused by overloading. The results have demonstrated that the plasticity caused by the loading part of the load cycle induces compressive residual stresses in the vicinity of the crack upon partial and/or full unloading. The experimental work has been carried out aiming to ascertain the constant load amplitude fatigue crack growth behaviour. It has been established that the material does not exhibit a sigmoidal crack growth curve but, one of parabolic like shape. A n empirical crack growth law has been derived to describe the constant load amplitude behaviour. It has been established that overloads cause substantial extension of the total life to failure due to delays caused by overloading. The observed delay increases if block overloads are introduced, and crack arrest is possible to occur . Finally, it is concluded that crack closure is responsible for the observed crack growth rate reductions.
Open Access
An experimental and theoretical study of flow in a vortex controlled diffuser
(1980-09) Saad, E. A; Adkins, R.
The flow inside a vortex controlled diffuser was studied experimentally and theoretically. The experimental side of the study included the following: (1) The selection of a suitable laser Doppler velocimeter for the measurement of the mean velocity and the Reynolds' stresses. (2) An experimental optimisation of the geometry of a diffuser model was conducted. The results were used in the design of the diffuser model which was later used for the measurement with the LDV. (3) The distribution of the statin pressures the components of the mean velocity and the Reynolds' stresses in the diffuser model were measured. The results wore presented and discussed. Tile analysis of the results established a comprehensive hypothesis about the flow mechanism. This enhanced the present knowledge about the flow behaviour inside this particular diffuser. (4) Suggestions for further development of the design of the diffuser were proposed.
Open Access
Fabrication of wavelength division multiplexed in-fibre Bragg grating arrays for structural monitoring applications
(1997-04) Dockney, Michael Lee; Tatam, Ralph P.
Wavelength division multiplexed arrays of in-fibre Bragg gratings (IFBGs) are fabricated in hydrogen loaded optical fibres. The IFBG arrays are embedded into a carbon fibre composite test beam for the purpose of quantitative dc strain sensing. A near-field phase mask writing technique is compared to a conventional mirror interferometer and a novel phase mask based interferometer writing scheme. The combination of a wavelength tuneable UV source and a phase mask based interferometer offers, either a large spectral coverage of 51nm, or a laser limited Bragg wavelength accuracy of 0.03nm in the 800nm spectral region. The characteristics of gratings fabricated using these schemes are discussed. The axial strain sensitivity of optical fibre is investigated in a preliminary experiment. A three point bend test is then used to compare the theoretical strain applied to a carbon fibre test beam with measurements made by electrical strain gauges and embedded IFBG sensors over the range ±3400pstrain. A residual tensile strain results from embedding and is generally measured to be >2000pstrain. The majority of IFBG sensors appear well bonded to the surrounding host material and, with one exception, the strain responses of uncoated IFBG sensors deviate from the theoretically predicted values by <4.3%. A buffer coat around the sensor reduces the strain response by ~4% but does not affect its linearity or reproducibility. A spliced IFBG pair is used to separate strain and temperature measureands. A transfer matrix for the scheme is experimentally determined that has a calculated condition number of 23. The technique is compared to other reported methods of strain and temperature separation. Also, a demodulation scheme for IFBG sensors based on a volume holographic filter formed in photorefractive BaTi03 is reported. The filter has a strain measurement range of 2500pstrain, with a minimum detectable strain of 4pstrain/VHz.
Open Access
FAR - field boundaries and their numerical treatment
(1989-07) Karni, S.; Roe, P. L.
Many computational problems of theoretical and practical interest are not naturally bounded by physical boundaries. Aerodynamic examples include flow calculations past aerofoils or past wing-body configurations, semi-bounded channel flows etc. Other examples include simulations of Turbomachinery flows, problems in Underwater Acoustics etc. To obtain a numerical solution, the problem has first to be converted to a finite region, by introducing an artificial boundary at some finite distance. Boundary conditions must be specified at the artificial boundary for well-posedness of the truncated problem. They should simulate an open boundary across which the fluid flows and should ideally allow outgoing waves to pass through without generating reflections. Indeed, reflections at the boundary not only degrade the accuracy of transient solutions but also inhibit convergence to steady-state. In many problems of practical interest, perfect absorption cannot be achieved. Instead one aims at minimizing the amount of reflected energy using asymptotic expansions based on various asymptotic arguments. The more accurate the boundary statements, the closer the artificial boundaries can be located to the regions of aerodynamic interest, thereby reducing the computational domain and costs. We present a thorough numerical study of the efficiency of several widely used boundary conditions in absorbing outgoing waves. We identify the key parameters upon which the level of absorption at the boundaries depends and expose the limitations of some of the existing recipes. We show that substantial reflections may occur even under conditions which are considerably milder than those encountered in practical calculations. We then introduce an unconventional approach to the treatment of artificial boundaries. It is proposed that in the far field the governing equations are modified in a boundary-layer like manner. Two closely related far field modifications are derived and analysed: (a) Slowing down the outgoing waves and (b) Attenuating the outgoing waves. Under the first modification the outgoing waves are prevented from reaching the boundary hence from reflecting. Under the second, the outgoing waves are attenuated to practically zero strength before reaching the boundary. Both modifications do not alter the propagation of the incoming waves to allow the launching of correct information from the boundary into the interior. Analytic conditions are derived to ensure that no reflections are generated due to the change of coefficients in the governing equations. Reflection analysis is also performed on the discrete level. Well-posedness of the modified systems is established as well as stability of the resulting interface problem. The modifications are extended to two space dimensions and are applied to a variety of one and multidimensional test problems. Results indicate that the proposed far field modifications are attractive in genuinely time-dependent calculations. Preliminary steady state calculations with the unsteady 2D Euler equations show significantly improved convergence properties.
Open Access
Fatigue fracture in the presence of shot peening residual stresses: initiation, propagation and relaxation phenomena
(1988-09) Hammond, D. W.; Meguid, S. A.
This thesis describes both experimental and theoretical work performed in order to analyse fatigue fracture in the presence of shot-peening induced residual stresses. In the study, four contrasting alloys were examined by fatigue testing and use was made of different test techniques to illustrate some of the phenomena associated with fatigue fracture in shot-peened components. The results demonstrate that in order to characterise the process of fatigue in a component bearing residual stresses, it is necessary to consider three groups of variables. These are the applied and residual stress state, the geometry of the component and the flaws it contains and the nature of the alloy itself, including any propensity to work harden, recover or undergo phase changes due to peening or fatigue loading.
Open Access
Finite element and boundary element methods for elasto-plastic stress analysis of two-dimensional and axisymmetric problems
(1989-07) Husain, Imad Ahmed; El-Zafrany, A.; Cookson, A.
This work dealt with elasto-plastic analysis of 2-dimensional and axisymmetric problems using the finite-element and boundary-element techniques. The analysis is based upon a vide range of yield criteria and hardening rules so as to be suitable for most engineering materials. Many accuracy measures have been employed with the boundary-integral equation derivations, such as isoparametric boundary elements, singular quadrature, corner models, finite-difference schemes for internal stress evaluation, high-order integration cells, and the use of boundary-integral expressions for domain loading terms. A programming package has been developed vith its own mesh generator and results plotter. The package has been validated using several case studies including a benchmark test case recommended by NAFEMS. The finite-element analysis of elasto-plastic problems proved to be more stable and reliable than the corresponding boundary-element analysis.
Open Access
Fluid flow analysis using the boundary element method
(1993-05) Carneiro, Helder Fernando De Franca Mendes; Ramsden, K. W.
An Attempt at development of theory and computer program for the solution of potential and viscous flow problems using the Boundary Element Methods is introduced in this work. A program for potential flow analysis of cascades is developed and properly validated. Theory and programs for boundary element analysis of viscous flow using vorticity-velocity formulation and pressure-velocity formulation along with the penalty function approach were developed. Many tests were carried out on the programs, and case studies of Poiseuille, Couette and driven-cavity flows were analysed and results were compared with existing solutions. The developed vorticity-velocity algortithms converge well for low Reynolds number, but the program based on the penalty function approach has limited success only for cases without domain integrations.